Activated carbon is widely used for recovering precious metals including gold and silver, throughout the precious metals industry. In one generally preferred prior-art gold recovery method, a reagent solution of potassium or sodium cyanide or some other chemical is used to dissolve a precious metal from its ores. Coarse carbon, for example, about 20-mesh or greater, is added to the gold-containing solution so formed. The dissolved precious metal can be effectively adsorbed on to the coarse carbon. The coarse carbon is then separated from the solution by passing the mixture of carbon and solution through 20-mesh screens.
The gold is then stripped from the gold-loaded carbon with a further reagent solution of sodium or potassium cyanide to form a second gold-containing solution. The gold is then generally recovered from the solution by an electrochemical process such as electrowinning.
One disadvantage of this carbon adsorption method is a loss of carbon, and gold adsorbed thereon, during the various steps of the process. This loss occurs as a result of a percentage of coarse carbon being reduced to fine carbon particles, for example less than about 20 mesh, during the above-described process steps. The particles are sufficiently fine that they can not be retained by the 20-mesh screens typically used for carbon separation. Because of this the fine carbon may be deposited in the tailing along with the adsorbed gold. A global survey of 36 selected gold producing companies indicated that the average carbon consumption in this process was 0.025 pound per ton of processed ore (D. Seymour, Carbon consumption in Precious Metal Recovery--An Industry Survey, Randol Proceedings, pp319-326, Vancouver, Canada, 1992). Yearly carbon consumption at the 36 mines surveyed is approximately 2,000 tons. This carbon can contain from about 4 to 50 ounces (oz) of gold per ton.
Due to the potential loss of gold on the fine carbon, most precious metal producing companies employ some method, for example, flocculation and filtration, to recover at least a part of the residual fine carbon generated during the process steps. In one industry standard procedure, once sufficient fine carbon has been recovered, the recovered fine carbon is shipped off site to be processed (Rescan Engineering LTD., World gold Survey, Section 4, May 1998) for recovery of any precious metal adsorbed thereon. The typical off-site process includes grinding and smelting the fine carbon, followed by recovering precious metals from the ash with further stages of cyanide dissolution and electrowinning.
The average cost of shipping and processing of the fine carbon can be sufficiently high that about five to six ounces of gold must be recovered per ton of fine carbon for the recovery process simply to break even. Clearly, there is a need for a simple and inexpensive on-site process for recovering precious metals from the fine carbon produced during the coarse carbon process for precious metal recovery.